The present invention generally relates to a method for forming an interlayer dielectric film in a multi-level inter-connection structure for a semiconductor integrated circuit device. More particularly, the present invention relates to a method for forming an interlayer dielectric film with a low dielectric constant by polymerizing an organic/inorganic hybrid material (e.g., a siloxane material with organosilicon bonds) within plasma.
Examples of the interlayer dielectric film made of a siloxane material with organosilicon bonds include: an organic SOG film formed by a coating technique; and a siloxane film with organosilicon bonds, which is formed by polymerizing an organosilicon compound within plasma.
An organic SOG film is usually formed in the following manner. First, the surface of a substrate is coated with a solution of a siloxane polymer with organosilicon bonds at room temperature to obtain a coating film. Next, the coating film is heated with a hot plate to vaporize the solvent of the solution. Then, the film is hardened at an elevated temperature of 400xc2x0 C. within an inert gas environment. During this hardening process, silanol (Sixe2x80x94OH) bonds, of which the siloxane polymer is made up, cause dehydration and condensation reactions to form a siloxane polymer. As a result, the organic SOG film is densified.
A siloxane film with organosilicon bonds may be formed by the plasma polymerization process in the following manner. First, an organosilicon compound and an oxidizing agent such as nitrogen monoxide are polymerized with each other by a plasma CVD process, thereby generating organic silanol. Then, the organic silanol bonds themselves are polymerized with each other to obtain a siloxane film with organosilicon bonds.
According to the known method for forming an organic SOG film, however, the solvent is vaporized from the coating by heating the coating. Thus, the solvent in the organic SOG film may not be removed completely, but left in the coating. In such a situation, an outgassing phenomenon, or gradual vaporization of the residual solvent from the organic SOG film, is observed during the heat treatment conducted after the film has been formed. Then, a contact hole cannot be filled in with a metal film satisfactorily due to the outgassing phenomenon. As a result, the resistance of a contact formed in this manner becomes higher than expected.
A similar phenomenon is also observable in forming a siloxane film with organosilicon bonds by a plasma polymerization process. Specifically, when a silanol polymer is formed as a result of the dehydration and condensation reactions of silanol bonds, unreacted silanol is left in the siloxane film. Thus, depending on the conditions of a thermal process during an integration process performed after the film has been formed, the dehydration and condensation reactions of the residual silanol proceed gradually. As a result, an outgassing phenomenon, i.e., vaporization of water produced by the dehydration and condensation reactions of the residual silanol, is also observed and the resistance of a contact formed in this manner rises, too.
In addition, if a siloxane film with organosilicon bonds is formed at 300xc2x0 C. or more by the plasma polymerization process, then the organosilicon bonds cannot be incorporated into the resultant film effectively. As a result, the dielectric constant of that film is not so low as expected.
To solve these problems, an alternative method of forming a siloxane film was proposed. According to this method, a siloxane film is formed at a temperature as low as room temperature, and then subjected to a special heat treatment at about 200xc2x0 C., thereby stabilizing the siloxane film obtained. This method is, however, impractical, because the temperature and environment should be controlled too precisely to execute the special heat treatment successfully.
It is therefore an object of the present invention to form an insulating film having a low dielectric constant with the outgassing phenomenon suppressed and without conducting any special heat treatment.
A first inventive method for forming an interlayer dielectric film includes the step of forming the interlayer dielectric film out of an organic/inorganic hybrid film by plasma-polymerizing a source material at a relatively high pressure within an environment containing nitrogen gas as a dilute gas. The source material includes an organosilicon compound.
According to the first method, the plasma polymerization is produced at a relatively high pressure within an environment containing nitrogen gas. Thus, the electron temperature of the plasma is controllable to a low temperature and the energy of the plasma can be consumed in exciting the nitrogen gas. That is to say, it is possible to suppress the organosilicon bonds from being decomposed by the plasma. As a result, the organosilicon bonds can be effectively incorporated into the organic/inorganic hybrid film and the dielectric constant of the resultant interlayer dielectric film can be reduced.
In addition, since the organosilicon bonds can be incorporated into the organic/inorganic hybrid film effectively, the creation of silanol, which usually causes the outgassing phenomenon, can be suppressed. Furthermore, the organosilicon bonds are more stable thermally than silanol, and are less likely to react irrespective of the conditions of a thermal process during the integration process after the film has been formed. Thus, it is possible to prevent the outgassing phenomenon from being produced in the interlayer dielectric film.
In the first method, the pressure is preferably 650 Pa or more.
Generally speaking, the pressure of a vacuum created differs depending on various process conditions including the temperature inside the reaction chamber of a CVD system, the temperature of a process gas and the volume of the reaction chamber. Thus, the process is preferably controlled by the residence time of the process gas, because the time is constant irrespective of the conditions such as these. Specifically, a one-to-one correspondence is definable between the residence time T of the process gas and the vacuum by the following conversion equation:
T=(volume of reaction chamber)/V2
=(volume of reaction chamber)xc3x97(P1/P2)xc3x97(T2/T1)xc3x97V1
where V1 is the flow rate of the process gas, V2 is the flow rate of the gas inside the reaction chamber, P1 is the pressure of the process gas, P2 is the partial pressure of the process gas inside the reaction chamber, T1 is the temperature of the process gas and T2 is the temperature inside the reaction chamber.
In the present invention, the volume of the reaction chamber was 127000 ml. The flow rate V1, pressure P1 and temperature T1 of the process gas were kept constant at 2000 ml/min., 101325 Pa and room temperature (=25xc2x0 C.), respectively. And the temperature T2 inside the reaction chamber was also kept constant at 200xc2x0 C. Since nitrogen gas was introduced as a dilute gas at 5000 ml/min., the partial pressure P2 of the process gas inside the reaction chamber can be calculated as two-sevenths of the vacuum. The following Table 1 shows the relationship between the vacuum and the residence time of the gas we obtained under these conditions:
As shown in Table 1, if the vacuum is 650 Pa or more, then the residence time of the process gas should be 0.178 minute or more.
Accordingly, if the vacuum has a pressure of 650 Pa or more, the electron temperature of the plasma is controllable to a low temperature just as intended and the organosilicon bonds can be incorporated into the organic/inorganic hybrid film with much more certainty. As a result, the dielectric constant of the interlayer dielectric film can be further reduced.
A second inventive method for forming an interlayer dielectric film includes the step of forming the interlayer dielectric film out of an organic/inorganic hybrid film by polymerizing a source material within plasma with a low electron temperature. The source material includes an organosilicon compound.
According to the second method, the organosilicon bonds can also be incorporated into the organic/inorganic hybrid film effectively. Thus, the dielectric constant of the interlayer dielectric film can be reduced and it is possible to prevent the outgassing phenomenon from being produced in the interlayer dielectric film.
In one embodiment of the first or second method, the organosilicon compound preferably has an Sixe2x80x94Oxe2x80x94Si bond.
In such an embodiment, the organosilicon bonds can be incorporated into the organic/inorganic hybrid film effectively, and the creation of silanol can be suppressed. Thus, the outgassing phenomenon is even less likely to occur in the interlayer dielectric film. In addition, the percentage of the siloxane bonds formed by the oxidizing agent becomes relatively low among all the siloxane bonds. Accordingly, the density of the organic/inorganic hybrid film can be lowered and the dielectric constant of the interlayer dielectric film can be further reduced.
In another embodiment of the first or second method, the organosilicon compound preferably has an Sixe2x80x94Oxe2x80x94R bond, where R is selected from the group consisting of alkyl, allyl and aryl groups.
In such an embodiment, the organic components can be incorporated into the organic/inorganic hybrid film effectively, and the creation of silanol can be suppressed. Thus, the outgassing phenomenon is much less likely to occur in the interlayer dielectric film. In addition, the percentage of three-dimensional siloxane bonds increases and the structure of the organic/inorganic hybrid film is even more similar to that of a pure silicon dioxide film. As a result, the mechanical strength of the organic/inorganic hybrid film increases. Furthermore, since the percentage of Sixe2x80x94O bonds, which can be easily and strongly bonded to various metal films or insulating films, increases, the adhesiveness of the film improves greatly.
In still another embodiment of the first or second method, the organosilicon compound may be selected from the group consisting of hexamethyldisiloxane, methyltrialkoxysilane, dimethyldialkoxysilane, trimethylalkoxysilane, tetramethylsilane and a mixture thereof.
In yet another embodiment of the first or second method, the plasma polymerization is preferably conducted at a temperature of 350xc2x0 C. or more.
In such an embodiment, the residual silanol can be reduced and the outgassing phenomenon is even less likely to occur in the interlayer dielectric film.
In still another embodiment, the plasma polymerization is preferably conducted within an environment in which an oxidizing agent is contained. Then, the organic/inorganic hybrid film can be formed with much more certainty.
In this particular embodiment, the oxidizing agent is preferably nitrogen monoxide.
Also, the mass of the oxidizing agent contained is preferably equal to or less than its chemical equivalent for the organosilicon compound. In such an embodiment, silicon, contained in the organosilicon compound, is less likely to contribute to the formation of a silicon dioxide film. Accordingly, the organosilicon bonds can be incorporated into the organic/inorganic hybrid film more effectively and the dielectric constant of the interlayer dielectric film can be further reduced.
In still another embodiment, the interlayer dielectric film is preferably formed by heating the organic/inorganic hybrid film to a temperature higher than a temperature set for the plasma polymerization.
In such an embodiment, volatile organic components can be incorporated into the organic/inorganic hybrid film and then vaporized therefrom by a heat treatment. That is to say, the interlayer dielectric film can have its porosity further increased and its density further decreased. As a result, the dielectric constant of the interlayer dielectric film can be further reduced.
In yet another embodiment, an organic compound is preferably contained in the source material.
In such an embodiment, the weight of the organic components incorporated into the organic/inorganic hybrid film increases. As a result, the dielectric constant of the interlayer dielectric film can be further reduced.